How much oil (in terms of millions of barrels) is still under the
major Texas oil fields? Compared to other major oil fields in the
world (such as the North Sea or the Gulf of Mexico), what quality is
Texas oil considered to be in the industry?

Hi Wilddemocrat,

Texas Petroleum Supply (Upstream) 

Crude Oil Proved Reserves: 4,613 million barrels (2004), ranked 1st
(2nd including Federal Offshore). Accounts for 22 percent of U.S.
crude oil proved reserves.
Crude Oil Production: 1,073 thousand barrels per day (2004), ranked
1st (2nd including Federal Offshore). Accounts for 20 percent of U.S.
crude oil production.
Total Producing Oil Wells: 151,653 (2004) 
Rotary Rigs in Operation: 506 (2004) 

http://tonto.eia.doe.gov/oog/info/state/tx.html